In our U.S. Pat. No. 4,751,113 we disclose methods of providing a layer of metal on an adhesion and seal layer and in our U.S. Pat. Nos. 4,618,504 and 4,521,475 we disclose the adhesion and seal layer incorporating micron-sized hollow glass or ceramic spheres or spaces formed by foaming (syntactic foams) which are abraded or blasted so as to rupture the outermost layer of spheres, beads or spaces to provide a plurality of pores, nooks, crannies and undercuts for receiving the thermally sprayed metal which becomes imbedded into the undercuts, pores, nooks and crannies such that the bond or adherent strength of the metal coating is greatly improved. Further, in our U.S. Pat. No. 4,751,113, for marine surfaces, we disclose that a light weight wet sanding of the exposed metal surface produced a smoother marine surface. However, this light wet sanding removes some of the sprayed metal, typically copper or copper based alloys.
The purpose of the present invention is to obtain a smoother and better hydrodynamic surface. The roughness of thermal spray deposits ranges nominally from 300 to 450 micro inch finishes. A paint overcoat can reduce this roughness to less than 100 micro inches. This will avoid the need of smoothing the coating for optimum hydrodynamic performance, if desired. This sealer overcoat also allows a rougher spray coat to be applied which can be deposited with greater efficiency and usually achieve a higher bond strength and simultaneously providing a better hold for the paint.
The seal overcoat can provide color which has been available to boaters for decades and would make it more acceptable in the marketplace where some emphasis is placed on the cosmetics or aesthetics of the finish as opposed to the functionality thereof.
The present invention is a base coated with antifouling copper alloy spray coating covered with the seal overcoat, and a method for preparing the coated structure. The seal overcoat according to the present invention provides a means of prolonging the already long term life span of the antifouling copper alloy spray coating by reducing the oxidation and corrosion coating that takes place during the first year of immersion. Studies indicate that the copper alloy can corrode at an accelerated rate during the first year and after the coating of corrosion products are formed, slows down to average 0.05 mils over a 5-year period but it can be as much a 2 mils the first. This is a 40 times greater than the ultimate average, hence, the seal overcoat, by permeating the surface layer of sprayed coating, since the coating may have up to 20% porosity, will suspend this accelerated corrosion and extend the life of the sprayed coating considerably.
The overcoat according to the invention will also provide at the water line, a screen from direct sunlight. Experiments have shown that the sunlight can adversely effect the corrosion rate of copper and copper alloys. As a shield, the overcoat can be a positive influence to the life of the coating along the water line. Although the antifouling overcoat paint does not have the longevity of the metal spray, by filling the valleys and porosity of the microscopic recesses in the metal sprayed coatings, it will enable the asperities of the metal sprayed coating to be initially exposed and corrode preferentially so that in effect, the "high spots" are subjected to the initial higher corrosion rate and thereby have a smoothing effect on the metal coating over a period of time.
A properly formulated antifouling paint contains no hazardous organotins and can be a valuable adjunct to the metal spray coating with respect to longevity and performance. The overcoat on the sprayed metal coatings may also provide protection against heavy galvanic corrosion. If a hull has some dissimilar metal exposed to the sea water although it may not be in contact with the metal spray coating, yet if electrically connected internally in the hull it can form a continuous path whereby the material, and if it is less noble to the copper/nickel coating, it will corrode very rapidly due to the large area of the copper/nickel vs. the anodic material.
An overcoat will prevent the immediate large current flow by acting as an insulator to the large metal surface. The gradual corrosion of the metal sprayed coating will not cause the highly accelerated attack on the ground metal sections. Ideally, the coating has to be electrically isolated from ground and other metal members of the hull when originally applied.
The properties of the sealed coat are that it has (1) a low surface tension to permit wetting and penetration of the sprayed metal coating surface and porosity. (2) it has leveling and flattening properties to produce a smooth finished surface. (3) some thickening agent to provide thixotropic properties on vertical surfaces. (4) Abalative properties to result in a uniform leaching rate. (5) High copper oxide content for antifouling. A typical formulation for the sealed paint coating is any high percent cuprous oxide or copper based antifouling paint.
The invention can be used with any abalative type antifouling paint diluted with any of the recommended thinners such as: copper napthanate 20%, amine salt of tetrachlorophene 1%, and mineral spirits 79% but not limited to.
Thus, rather than abrading the surfaces by the light wet sanding or other mechanical smoothing operation, the thermally sprayed antifouling surface is painted with an antifouling paint formulated to make the surface "smooth to the touch". The paint has a strong bond by filling in all the valleys and interstices of the sprayed metal coating and has the additional advantage of prolonging the life of the metal coating.